Surface finishes are applied over copper conductor layers of printed circuit boards (PCB). A patterned surface finish may act as a mask to protect a selected pattern of copper conductor on a substrate during etching of the copper. The surface finish can also reduce or eliminate corrosion of the copper surface to ensure suitable surface chemistry for the application of and reaction with solder for making electrical and physical bonds with electrical components.
Additionally, some surface finishes are especially good at providing a smooth surface. A smooth surface is especially important for mounting high-density components in high value products such as cell phones, tablets, and laptop computers. One currently available surface finish that is especially attractive for high density component mounting is Electroless Nickel/Immersion Gold, commonly referred to as ENIG.
Unfortunately, ENIG has a drawback in that it is prone to forming brittle solder joints. In some cases, a condition referred to as “black pads” has been found to correspond to brittle solder joints. Brittle solder joints can fail (especially under vibration and/or shock load) and lead to malfunction and failure of the electronic assembly and the product in which the electronic assembly operates.
FIG. 1 is a (not to scale) side sectional diagram of an ENIG surface treatment, according to the prior art. As may be appreciated, while one principal benefit of ENIG is intended to be smoothness of the surface, deep intergranular boundaries 112 in the electroless nickel 106 tend to cause openings in the gold layer 110 and a surface roughness that may be less than ideal.